Hard disk drives are used in almost all computer system operations. In fact, most computing systems are not operational without some type of hard disk drive to store the most basic computing information such as the boot operation, the operating system, the applications, and the like. In general, the hard disk drive is a device which may or may not be removable, but without which the computing system will generally not operate.
The basic hard disk drive model was established approximately 50 years ago and resembles a phonograph. That is, the hard drive model includes a storage disk or hard disk that spins at a substantially constant rotational speed. An actuator arm or slider is utilized to reach out over the disk. The arm has a head-gimbal-assembly (HGA) composed of a suspension, flexure and a slider carrying the read/write components.
In operation, the hard disk is rotated at a set speed via a spindle motor assembly having a central drive hub. Additionally, there are tracks evenly spaced at known intervals across the disk. When a request to read a specific portion or track is received, the actuator and servo-system of the hard drive aligns the head, via the arm, over the specific track location and the head reads the information from the disk. In the same manner, when a request to write to a specific portion or track is received, the hard disk aligns the head, via the arm, over the specific track location and the head writes the information to the disk.
Over the years, the disk and the head have undergone great reductions in their size. Much of the refinement has been driven by consumer demand for smaller and more portable hard drives such as those used in personal digital assistants (PDAs), MP3 players, and the like. For example, the original hard disk drive had a disk diameter of 24 inches. Modern hard disk drives are much smaller and include disk diameters of less than 2.5 inches (micro drives are significantly smaller than that). Advances in magnetic recording are also primary reasons for the reduction in size.
A second refinement to the hard disk drive is the increased efficiency and reduced size of the spindle motor spinning the disk. That is, as technology has reduced motor size and power draw for small motors, the mechanical portion of the hard disk drive can be reduced and additional revolutions per minute (RPMs) can be achieved. For example, it is not uncommon for a hard disk drive to reach speeds of 15,000 RPMs. This second refinement provides weight and size reductions to the hard disk drive, it also provides a faster read and write rate for the disk thereby providing increased speed for accessing data. The increase in data acquisition speed due to the increased RPMs of the disk drive and the more efficient read/write head portion provide modern computers with hard disk speed and storage capabilities that are continually increasing.
However, the higher RPMs of the disk have resulted in problems with respect to the interaction of the air with components of the hard disk drive. For example, although the hard disk drive is closed off from the outside, it has an amount of air within its packaging. As the disk spins and the RPMs increase, the air within the hard disk drive package will also begin to rotate and will eventually approach the speed at which the disk is rotating especially near the spindle hub and disk surfaces. This is due to the friction between the disk and the air. In general, Reynolds numbers are used to represent the flow characteristics. For example, in one case the Reynolds number may be based on the tip speed of the disk. That is, the linear velocity at the outer diameter of the disk.
Only when the Reynolds number is sufficiently small (e.g., an enclosure with reduced air density), the air may stay in laminar flow with the boundary layer of air remaining smooth with respect to the rotating disk. However, any obstructions to the flow will result in turbulence. That is, due to the introduction of obstructions to the airflow at large Reynolds numbers (e.g., typically several thousands based on the disk-to-disk spacing and the local disk circumferential velocity), the airflow will become turbulent as it passes the obstruction.
As is well known from fluid mechanics, the characteristics of hard drive components placed in turbulent airflow can include buffeting, harmonic vibration, and the like. Each of these characteristics will result in problematic motion for the arm and head portion and/or the rotating disk. The problematic motion will result in excessive track misregistration (TMR). This is even more significant as the tolerances are further reduced.
Data is stored on the disks on a series of concentric circles that are also referred to as “servo tracks” which are assigned a track number so that the HGA can locate a specific track on a given disk. A servo system controls the HGA to move the read/write head over a defined track on a given disk and maintains the read/write head over the defined track, typically based upon data that is read from a servo track written onto the disk.
This data is written onto the surface of the disks in a process called “servo writing.” One servo writing process uses a machine called a “servo writer” to mechanically position the actuator at a desired track position where servo track position data is then written onto the disks. In another process, called “self servo writing,” the hard disk drive itself is used to position the actuator when writing the tracks. Typically, the actuator is moved to the position of the disk closest to the spindle and a track position data is written. The servo motor moves the arm laterally to a position corresponding to the next track and the servo track position data is written for that track. It is important during the servo track writing process to minimize vibration or other disturbances to the actuator so that the actuator can be precisely positioned in a stable manner. Otherwise, irregularly shaped (e.g., not round) servo tracks are written onto the disks.
Every disk drive created has a TMR budget and allowances to compensate for aerodynamic buffeting of the actuator are a significant portion of the TMR budget. Thus, a greater allowance for TMR (e.g., due to aerodynamic buffeting) results in increased track pitch and reduced area density on a given disk. As the data storage industry strives for innovations to improve area density of hard disk drives, it is important to reduce the pitch between successive servo tracks on the disks.